RCD Testing in Detail

<h2>Why RCDs need testing</h2>
<p>RCDs are mechanical and electronic devices that can fail or become sluggish over time. Periodic testing verifies they still operate correctly within the required disconnection times. Unlike MCBs which operate on overcurrent, RCDs must be tested using a current that simulates a real earth leakage fault.</p>

<h2>Test sequence and requirements</h2>
<table>
<tr><th>Test</th><th>Test current</th><th>Required result</th></tr>
<tr><td>Non-trip test</td><td>½ × IΔn</td><td>Must NOT trip</td></tr>
<tr><td>Operating time</td><td>1 × IΔn</td><td>Trip within 40ms</td></tr>
<tr><td>High current</td><td>5 × IΔn</td><td>Trip within 40ms</td></tr>
<tr><td>Type S at rated</td><td>1 × IΔn</td><td>Trip within 500ms</td></tr>
</table>

<h2>RCD types and their applications</h2>
<ul>
<li><strong>Type AC</strong> — sinusoidal AC fault currents only. Not suitable where electronics are connected.</li>
<li><strong>Type A</strong> — AC and pulsating DC. Required for washing machines, dishwashers, EV chargers, PV systems. This is now the minimum for most domestic circuits (18th edition).</li>
<li><strong>Type B</strong> — AC, pulsating DC and smooth DC. Required for some three-phase drives and specialist EV chargers.</li>
<li><strong>Type S</strong> — Time-delayed (selective). Used as main switch for discrimination. Max trip time 500ms.</li>
<li><strong>Type F</strong> — For variable frequency drives.</li>
</ul>

<h2>The integral test button</h2>
<p>All RCDs have a test button that simulates a fault by passing a small current through a resistor. This tests the mechanical trip mechanism but NOT the sensitivity of the electronic detection circuit. The test button should be pressed monthly by the user — but full instrument testing is required at inspection intervals.</p>

<h2>Key exam points</h2>
<ul>
<li>½ × IΔn: must NOT trip</li>
<li>1 × IΔn: must trip within 40ms (general purpose), 500ms (Type S)</li>
<li>5 × IΔn: must trip within 40ms</li>
<li>Type A now required for most circuits (Regulation 531.3.3)</li>
<li>Test button tests mechanism only — not electronic sensitivity</li>
</ul>